PJSIP: Prevent crash on blind transfer

Blind transfers don't go too well with NULL channels which can occur if
the channel has already been transferred away.

(closes issue ASTERISK-23718)
Reported by: Jonathan Rose


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@414948 f38db490-d61c-443f-a65b-d21fe96a405b